I think I think that he should set up a date without really thinking.
Louis freaks out and in a panic, runs out to get the clothing/make up/etc etc. to dress up as Louise.
The big day comes and it all goes pretty well, Pierre has a few 'wtf is up with this chick moments' and Louis spazes a bit, but again, it all goes over pretty well... until they get back to Pierre's place.
He tries to put the moves on 'Louise' only to have one of whatever he stuffed his bra with fall out or his wig could slip off or something.
This could go two different directions:
The typical yaoi manga fashion: Pierre suddenly realizes he has the hots for Louis when the poor boy confesses to him that he lied because he loved him in a typical teary-eyed uke fashion
oooooor
The more realistic approach: Pierre has a tissy fit and tells Louis that he never wants to see the lying brat again, despite any and all explanation, and Louis has to work himself into Pierre's good graces again in his own gender.
8D
Yes, no, maybe so?

I agree with ewroin and Pawky!, Louis in drag since it is something I can so see him going..."that would be a good idea! that would work!" haha. I mean he took Mr Hobos advise for starters. I could see him going there in drag acting all embarrassed.

Also that Pierre during the date realises this person is more and more like Louis and then figuring it out, or something happening to make it obvious Louise was Louis. Due to his reactions in the art class I think he is already partly there with figuring out they are the same person, just at the moment a bit to blinded with love to see it clearly.

I have a funny and crazy idea! it would probably not work at all~<3
If I remember correctly, Louis is an only child...

He could use this as a real trigger-test for Pierre!!!<3

He could go in "Louise"'s place and say they're related (twins? lame? whatever)
He apologizes and acts surprised all "So YOU'RE the Pierre that Louise was talking about!"
and Pierre's all "Ah, and you're her brother! ((So I'm not crazy! he really did look like her!!))"

but Pierre already ordered food! D: so Pierre says that to apologize for Louis having to go all the way here, he'd treat him to dinner! ohohoho...

and then Louis tries to see if he can trigger Pierre's memories of their shared childhood! MUAHAHA!